It is known a fuel vapor treatment apparatus that causes a canister to temporarily adsorb fuel vapor produced in a fuel tank and introduces the fuel vapor desorbed from the canister as required into an intake passage of an internal combustion engine to purge the fuel vapor. In the fuel vapor treatment apparatus, a leak detecting apparatus is provided in order to detect a leakage of fuel vapor leaking from an evaporation system into an outside of the system.
In the leak detecting apparatus shown in JP-2004-232521A (US-2004-149016A), a pump is connected to a canister through a measure-passage. While the pump decompresses an interior of the evaporation system, a leak detection is performed based on a pressure in the measure-passage.
When the adsorbed amount of the fuel vapor is close to an upper limit of the canister adsorbing capacity, the fuel vapor is desorbed from the canister and is introduced into the pump. This is referred to as a blow-by of fuel vapor, hereinafter. When the blow-by of fuel vapor is arisen, the blow-by fuel vapor is sucked into the pump and then is discharged into outside of the pump. In the case where a discharge port of the pump is opened atmosphere, the leak detecting apparatus generates the leakage of the fuel vapor